The invention relates generally to data communications and more particularly to high speed data transmissions over the public switched telephone network.
The sudden popularity of the Internet as a communication tool has led to an intense push for higher data transmission rates over the Public Switched Telephone Network (PSTN). As a result, the demand for increased data transmission rates over analog twisted pair wiring is at an all time high. The most recent widespread standard is xe2x80x9c56Kxe2x80x9d analog modem technology developed by U.S. Robotics and Rockwell/Lucent. While these technologies generally have not generated true 56kbps performance under typical subscriber line conditions, they do provide a boost in performance from the previous standard of bidirectional 33.6kbps.
Theoretically, a connection of 64kbps should be attainable between the subscriber and the Internet Service Provider (ISP) via a standard Plain Old Telephone Service (POTS) connection. This is because 64kbps is the rate at which data is transferred from the Central Office (CO) linecard to the ISP or other remote terminal. Several factors prevent this from happening including imperfect line conditions and varying local loop lengths common to POTS analog networks. The primary reason, however, for this less than the theoretical transmission rate is that the PSTN was designed to carry voiceband frequencies in the range of 300 Hz-3.4 kHz.
With the advent of digital voice systems, the decision was made to use a xe2x80x9ccompandedxe2x80x9d (compressed/expanded) data to reduce the number of bits per digital sample from a nominal 13-bits to 8-bits. Companding schemes use higher resolution at low signal amplitudes and lower resolution at high amplitudes. Companded signals are suitable for voice frequencies but not for analog modems since they limit their apparent bandwidth to a ceiling of 33.6kbps. In practice, most analog modems are only able to achieve rates of 46-48kbps downstream due to less-than-perfect analog line conditions.
The Analog-to-Digital (xe2x80x9cA/Dxe2x80x9d ) portion of the linecard coder/decoder (xe2x80x9ccodecxe2x80x9d) is where the analog signal is converted to its 8-bit companded representation. Hence, the linecard codec acts as a bottleneck in the entire data communications chain. One way of avoiding this bottleneck is by removing the A/D conversion in the downstream direction. This is accomplished by requiring a digital connection between the provider and its CO and increasing the data throughput of the modem signals to capitalize on the extra capacity. This is the basis of 56k technologies.
Moreover, while the use of 56K standards results in downstream data throughput of 56kbps under ideal local loop conditions, the upstream direction must still contend with an A/D conversion into 8-bit companded data and is still limited to 33.6kbps. Imperfect conditions in the analog local loop further degradate the signal resulting in less than the 56/33.6kbps maximums.
Additionally, while 56K standards offer improvements over the older V.34+ standard, bandwidth is still needed to keep pace with upcoming technologies such as video conferencing, remote server access, and other high rate transmission protocols. If higher data throughput is to be achieved, the limitations in the CO need to be overcome. Overhauling the PSTN by replacing the 8-bit companded data scheme could solve the problem, but this is not a feasible solution since the cost of such as effort would be enormous.
The invention overcomes the limitation in bandwidth of prior communications standards including 56K by offering increased transmission rates using an analog modem communicating over the PSTN.
In one embodiment, an improved linecard device is disclosed. The linecard permits increased rate communications between a subscriber and a service provider over the PSTN. An analog interface couples the subscriber modem to the PSTN over a twisted pair connection from the subscriber""s modem. The linecard incorporates a digital interface to the digital backplane leading to the service provider""s modem. A converter is interspersed between the analog interface and the digital interface. A linecard microcontroller is configured to request bandwidth on the digital backplane.
According to one embodiment, the linecard incorporates a codec with a pattern recognition mechanism that receives code patterns from the service provider modem via the digital backplane. The code patterns from the service provider modem are decoded by the mechanism and if a predetermined code pattern is detected, a strobe signal is transmitted to the linecard microcontroller which interfaces with the digital backplane to request bandwidth.
In one embodiment, a code recognition function monitors the Pulse Code Modulated (PCM) input from the provider modem in the downstream direction. A certain amount of intelligence is employed in the code recognition mechanism to handle simple handshaking and act on the PCM codes received. The general instruction architecture places the provider modem in the master position and the codec in the slave position. The code recognition mechanism provides a way to dynamically allocate and deallocate timeslots during data communications based on code patterns received from the provider modem.
According to another embodiment, the linecard microcontroller can request more or less timeslots from the network administrator based on the code patterns received from the service provider or the subscriber. During periods of inactivity, the timeslots are deallocated to make room for other connections on the same backplane.
According to another embodiment, the codec includes a strobe terminal that permits sending interrupts to the linecard microcontroller for xe2x80x9con-the-flyxe2x80x9d timeslot allocation.
A technical advantage of the invention is that it provides the subscriber with much more bandwidth than currently available with analog modems while maintaining much of the same equipment and connection methods.
Another technical advantage of the invention is that it eliminates the need to employ a direct customer interface with the CO and thus no equipment needs to be installed at the subscriber""s residence. Thus, the invention allows the telecom infrastructure to be built up gradually accommodating other high rate communications protocols to support additional traffic.
Yet another technical advantage of the invention is that it permits replacement of the existing linecard in the CO with the linecard of the present invention enabling hardware and software changes at the CO to provide the increased bandwidth.